The present invention relates to an insulated gate field effect transistor (IGFET), particularly to an IGFET which has been subjected to a selective oxidation process with a silicon nitride (Si.sub.3 N.sub.4) film used as a mask.
An IGFET which has been subjected to selective oxidation using a Si.sub.3 N.sub.4 film as a mask suffers to a low extent from the parasitic channel effect, since the silicon dioxide (SiO.sub.2) film on the areas other than the gate is sufficiently thick. Also, when the process is applied to an integrated circuit (IC), certain results are excellent, such as the integration density which can be much improved, and occurrences of a discontinuity in the wiring are reduced. However, the selective oxidation process simultaneously brings about the possibility of a dielectric breakdown of the insulated gate film at a low voltage, when the electric field between the substrate and the gate electrode is applied to the insulated gate film. Namely, such an IGFET has the problem that the breakdown voltage of the insulated gate film is lowered by the above-mentioned selective oxidation process.
FIGS. 1-4 show the formation of the field oxide film by the selective oxidation in a conventional IGFET production process, and the causes of the lowered breakdown voltage of the insulated gate film which are currently considered.
At first, as indicated in FIG. 1, the silicon dioxide film 2 is formed with a thickness of about 500 .ANG. on the entire surface of the single crystal silicon substrate 1, and then the silicon nitride film 3 is selectively formed with a similar thickness on said SiO.sub.2 film 2. Next, as indicated in FIG. 2a, the field oxide film 22 is caused to grow to a thickness of about 8000 .ANG. on the single crystal silicon substrate 1, except at the gate portion, under a high temperature oxidizing atmosphere containing water vapor. However, in this selective oxidation process, the H.sub.2 O in the water vapor and the Si.sub.3 N.sub.4 in the film 3 react chemically as expressed by the reaction (1), producing NH.sub.3. EQU Si.sub.3 N.sub.4 +6H.sub.2 O=4NH.sub.3 +3SiO.sub.2 ( 1)
The ammonia (NH.sub.3) easily passes through the SiO.sub.2 film and therefore the NH.sub.3 generated by the reaction (1) reaches the area under the SiO.sub.2 film 2 of the gate portion and then reacts with the single crystal silicon substrate 1 in accordance with the reaction (2), producing Si.sub.3 N.sub.4 at the regions 21, 23. EQU 3Si+4NH.sub.3 =Si.sub.3 N.sub.4 +6H.sub.2 ( 2)
Among the regions of Si.sub.3 N.sub.4 produced by the reaction (2), the region 21 at the boundary of the single crystal silicon substrate 1 under the end of the Si.sub.3 N.sub.4 oxidation resistant mask 3 is called the "White Ribbon". FIG. 2b which shows an enlarged view of this part of FIG. 2a, where the Si.sub.3 N.sub.4 region 21 is generated by the seepage of H.sub.2 O from the end of the thick SiO.sub.2 film. Detailed explanation is omitted here since by E. Kooi et al in the Journal of the Electro-Chemical Society, Vol. 123, at page 117 (1976). In addition, it is explained by Kowada et al in the Journal of Japanese Applied Physics, Vol. 17, No. 4, at page 737 (1978) that the breakdown voltage of the insulated gate film is lowered not only by the Si.sub.3 N.sub.4 region 21 at the end of the gate region, but also by the Si.sub.3 N.sub.4 regions which have a possibility of existing in the center region of the gate. The reason is considered to be that the NH.sub.3 generated by the reaction between H.sub.2 O and Si.sub.3 N.sub.4, because of a crystal defect such as a pin-hole in the Si.sub.3 N.sub.4 film 3, further reaches the Si substrate after passing through the lower SiO.sub.2 film 2, producing the Si.sub.3 N.sub.4 region 23. In addition, as indicated in FIG. 3, in the ordinary IGFET, the Si.sub.3 N.sub.4 film 3 and the SiO.sub.2 film 2 are removed after the selective oxidation, but the Si.sub.3 N.sub.4 regions 21, 23 that formed during the selective oxidation still remain. This is mainly because the Si.sub.3 N.sub.4 regions 21, 23 may not exist as simply films, but they may be combined in a complicated manner with the impurity particles contained in the area near the surface of the single crystal silicon substrate 1, which may act as nuclei. Namely, since the insulated gate film is formed while these nitrides remain, a homogeneous film thickness cannot be obtained and as a result the breakdown voltage is lowered. In the case of an ordinary IGFET, the SiO.sub.2 to be provided between the gate electrode and the silicon base plate must be formed as thin as possible in order to provide a large electrostatic capacitance to the gate. However, when the SiO.sub. 2 film becomes too thin, the breakdown voltage is drastically lowered. Such lowering of the breakdown voltage is a major barrier for improving the characteristics of the IGFET.
A method involving the lowering of the reaction temperature at the time of the selective oxidation is proposed by B. W. Ormont et al in the Summary of Papers of the Electro-Chemical Society Spring Meeting (Boston), Abstract No. 89, at page 231 (1979), as the means for avoiding such lowering of the breakdown voltage in the insulated gate film due to the selective oxidation.
However, methods involving the lowering of the reaction temperature are inadequate for the actual process because an oxidation time of about 5 hours is usually required for obtaining a field oxide film of about 8000 .ANG. at a temperature of 1100.degree. C., and about 13 to 14 hours are required at a temperature of 950.degree. C. Moreover, B. W. Ormont et al also propose a method of using a thick Si.sub.3 N.sub.4 film as the oxidation resistant mask. However, it is undesirable to make the Si.sub.3 N.sub.4 film thick enough for preventing lowering of the breakdown voltage, because the stress applied on the base plate at the time of the selective oxidation increases.
Therefore, a method is needed for preventing the lowering of the breakdown voltage of the gate insulating film of an IGFET even when the selective oxidation is performed using a Si.sub.3 N.sub.4 film as a mask.